1. Field of the Invention
The present invention relates to image processing which takes into consideration the appearance of a scene and the appearance of an image.
2. Description of the Related Art
The percentage of homes in Japan with digital cameras at the present time has exceeded 50% (2004 Cabinet's Survey of Consumption Trends), and taking pictures by digital camera has become commonplace.
When a certain scene is captured by a digital camera outdoors, etc., there are cases where the luminance range of the subject (or scene) is broader than the luminance range that is capable of capture. In such cases, a subject that is outside the luminance range that is capable of capture is such that its tone information cannot be recorded, giving rise to so-called highlight-detail loss and shadow-detail loss. For example, when the picture of a person is taken outdoors in fair weather with exposure made to conform to the person, highlight-detail loss occurs in image areas of background sky or clouds, or shadow-detail loss may occur in image areas in the shadows of trees. On the other hand, human vision has a characteristic called “local adaptation” characteristic whereby the state of adaptation is changed over in response to the brightness of a viewed area so that brightness and color are perceived, thus enabling tones to be perceived in both bright and dark areas. This means that there are cases where the impression received when a real-world scene is viewed directly and the impression received when the captured image of this scene is viewed differ.
The high-dynamic-range imaging (HDR) technique is one technique for solving this problem. The HDR technique is broadly classified into an HDR capture technique and HDR reproduction technique.
The HDR capture technique is a technique that expands the dynamic range of capture in such a manner that highlight-detail loss and shadow-detail loss will not occur, and one known method thereof involves combining images captured by a plurality of exposures. An image acquired by the HDR capture technique will be referred to as an “HDR image” below.
The HDR reproduction technique is a technique in which an HDR image having a broad dynamic range is favorably reproduced by a display device or output device (the latter in the discussion below) having a narrow dynamic range. Here dynamic range is compressed in order to accommodate a disparity between the two dynamic ranges. Various methods have been proposed thus far with regard to the compression of dynamic range.
For example, iCAM06 proposed by J. Kuang (see Kuang, J., Johnson, G. M., Fairchild M. D. “iCAM06: A refined image appearance model for HDR image rendering”, Journal of Visual Communication, 2007) is a dynamic-range compression method that is based upon the concept of using an output device to reproduce an impression received when a scene is viewed. iCAM06 technique first simulates brightness and color, which a human has perceived in a real-world scene, processing with an HDR image. Next, the technique converts brightness and color of HDR image to brightness and color reproducible by an output device or display device, and finally converts these to signal values of the output device or display device. The simulation necessitates a “human vision model” obtained by modeling the mechanism by which a human perceives brightness and color. In order that brightness and color perceived by a human may be simulated more accurately, iCAM06 uses a human vision model that reflects the local adaptation mentioned above.
In order to simulate the impression of a viewed scene from an HDR image taking local adaptation into consideration, it is necessary to set an area (referred to as “size of adaptive visual field”) in which local adaptation is produced. In iCAM06, information as to how a real-world scene was viewed by a human is indefinite and the size of the adaptive visual field is set uniformly. In other words, regardless of the image, the size of the adaptive visual field is given as a ratio with respect to image width (e.g., 50%). With iCAM06, therefore, there are cases where the impression of a subject at the time of actual capture cannot be simulated correctly.
Further, in a case where a reproduced image (a monitor image or printout) is viewed as well, local adaptation should occur in a manner similar to that at the time of image capture. However, iCAM06 does not take into consideration local adaptation with regard to a reproduced image.